The cardinals almost always choose one of their own as the new pope, and then step out onto a balcony before the public and introduce him. Suppose they choose a non-cardinal who is far from Rome. Do they wait until his arrival in Rome to tell the public who he is? Do they wait until his arrival to announce that they’ve chosen someone? Might Swiss guards meet him at his residence in his home town and escort him to the airport to go to Rome?
(If they announce they’ve chosen someone but don’t say who he is until his arrival at the Vatican, I suspect journalists employing detectives would figure it out before the announcement. In April 2005 the first notice some journalists had that a pope had been chosen was a cell phone call from one of the musicians who performed after the new pope’s speech, saying the musicians had just received the call that it was time for them to report to the Vatican. There are lots of ways for information to leak out when it’s a big event.)